The present invention relates generally to roller finger followers used in internal combustion engines. More particularly, the present invention relates to a roller finger follower rocker arm device that accomplishes cam profile switching in an internal combustion engine.
Historically, the efficiency, emissions, and performance of internal combustion engines have been adversely limited by fixed valve lift profiles, i.e., valve lift profiles wherein the timing of the opening and closing of the valves is fixed relative to the angular position of the engine crankshaft and the amount of lift imparted to the valves is also fixed. By fixing the valve lift profiles of the engine, inherent compromises were made between low-speed operation (idle) and high-speed operation for peak power. Importantly, engines having fixed valve lift profiles must incorporate a throttle device to control the airflow (and output) of the engine. Throttle devices introduce large throttling losses (pumping work) and greatly reduce the efficiency of the engine, and also negatively impact emissions of oxides of nitrogen (NOx) and hydrocarbons (HC).
In contrast, modern internal combustion engines may utilize one of several methods and/or devices to vary the valve lift profile to, at least in part, control the flow of gas and/or air into and/or out of the engine cylinders. One such method is two-step cam-profile switching, wherein the engine valves, usually the intake valves, are actuated by a selected one of two valve lift profiles. Typically, the two valve lift profiles consist of a high-lift long-duration lift profile designed to provide high power output at high engine operating speeds, and a low-lift short-duration lift profile that is designed for high efficiency and low NOx emissions at low operating speeds.
Selection of, or switching between, the valve lift profiles is accomplished by a cam-profile-switching device, such as, for example, a mode-switching or two-step roller finger follower (RFF). Generally, a two-step RFF includes a body and a central roller that is selectively coupled to and decoupled from the RFF body by a shaft. The central roller is engaged by a first cam lobe of the engine camshaft. When the shaft carrying the central roller is coupled to the RFF body, engagement of the first cam lobe with the central roller causes the RFF body to pivot thereby actuating an associated engine valve according to the lift profile of the first cam lobe. When the shaft is decoupled from the RFF body, engagement of the central roller by the first cam lobe does not cause the RFF body to pivot. Rather, the shaft and central roller reciprocate relative to the RFF body thereby absorbing the motion of the first cam lobe. The body of the RFF, or a pair of outer rollers affixed to opposite sides of the RFF body, is engaged by a corresponding pair of second cam lobes. When the shaft carrying the central roller is decoupled from the RFF body, the RFF body is pivoted according to the lift profile of the pair of second cam lobes. Typically, the first cam lobe is the higher lift cam, and the pair of second cam lobes are zero-lift or low-lift cam lobes. Such a two-step RFF is more fully described in commonly-assigned U.S. Pat. No. 6,467,445, which issued Oct. 22, 2002.
Two-step cam profile switching systems are relatively simple and are operable over a relatively wide range of engine operating speeds. Further such systems are relatively easy to package on new and even existing engines. By operating the two-step cam-profile-switching mechanism in conjunction with a cam phaser a wide range of variation in the valve lift characteristic is obtained. Although such two-step variable valve actuation (VVA) systems achieve a relatively wide range of variation in the valve lift profile, they nonetheless represent a tradeoff between mechanical simplicity and less than continuous variation they provide relative to the mechanical complexity yet full variation that a continuously-variable VVA system provides. Two-Step VVA systems also require cam phasers having a wide range of authority and high or fast response rates in order to achieve the full benefit of these systems.
Therefore, what is needed in the art is a cam-profile-switching system that enables an increased and relatively wide range of variation in the valve lift profiles, and yet is relatively simple.
Furthermore, what is needed in the art is a cam-profile-switching system that provides an increased and relatively wide range of variation in the valve lift profiles over a relatively wide range of engine operating speeds.
Moreover, what is needed is a method of cam-profile-switching that achieves an increased and relatively wide range of variation in the valve lift profiles, and does so with conventional cam phasers having conventional cam phaser rates.
The present invention provides a three-step cam-profile-switching roller finger follower.
The present invention comprises, in one form thereof, a body with low-lift, high-lift and medium-lift body sections. Low, high and medium cam followers are carried by the low-lift, high-lift and medium-lift body sections, respectively. At least one locking assembly selectively couples together and decouples the low and high-lift body sections, and selectively couples together and decouples the low and medium-lift body sections.
An advantage of the present invention is that an increased range of variation in the valve lift profile is achieved with relative mechanical simplicity.
Another advantage of the present invention is that an increased range of variation in the valve lift profile is achieved across an increased range of engine operating speeds.
Yet another advantage of the present invention is that full potential of the system is achieved with conventional cam phasers having conventional cam phaser rates.